Constrained and unconstrained movements INVOLVE DIFFERENT control
strategies.
Desmurget, Michel, Michael Jordan, Claude Prablanc, and Marc Jeannerod.
Vision et motricite, INSERM U94, 16 av. du Doyen Lepine, 69500 Bron.
France, Dept of Brain and Cognitive Sciences, M.I.T, Cambridge, MA 02139.APStracts 3:0250N, 1996.
ABSTRACT
This experiment was carried out to test whether or not the rules governing the
execution of compliant and unconstrained movements are different (a compliant
motion is defined as a motion constrained by external contact). To answer this
question we examined the characteristics of visually directed movements
performed with either the index fingertip (unconstrained) or a hand held
cursor (compliant). For each of these categories of movements, two
experimental conditions were investigated: no instruction about hand path, and
instruction to move the fingertip along a straight line path. The results of
the experiment showed that: 1) The spatio-temporal characteristics of the
compliant and unconstrained movements were fundamentally different when the
subjects were not required to follow a specific hand path. 2) the instruction
to perform straight movements modified the characteristics of the
unconstrained movements, but not those of the compliant movements. 3) the
target eccentricity influenced selectively the curvature of the
"unconstrained-no path instruction" movements. Taken together, these results
suggest that compliant and unconstrained movements involve different control
strategies. Our data support the hypothesis that unconstrained motions are,
unlike compliant motions, not programmed to follow a straight line path in the
task space. This observation provide a theoretical reference frame within
which some apparently contradictory results reported in the movement
generation literature may be explained.

Received 9 September 1996; accepted in final form 22 November 1996.
APS Manuscript Number J720-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996